Muscle tissue offers the experimentor a range of metabolic capabilities (fast-twitch glycolytic, slow-twitch oxidative, and cardiac highly oxidative cells), and rates of energy utilization which may vary as much as 50 times during contractile activity compared to the basal rate. With the view that there is an overall unity in the mechanisms for integrating cellular energetics in the different types of cells (however different they may be in detail), we will use a comparative approach to measure quantitatively mechanisms affecting muscle energetics. What is the role of intracellular pH in cellular energetics? How well can we quantify the unidirectional fluxes of the reaction catalyzed by creatine kinase in different cell types? We will test whether the creatine kinase reaction is reversible during contraction as well as at rest, and develop quantitative models to explain the results and to test concepts of metabolic compartmentalization between myofibrils, cytosol and mitochondria. I will use slowly-metabolized creatine analogues to manipulate the system further: what is the basis for what appears to be the increased aerobic metabolic adaptation? How does the free energy available from the coupled hydrolysis of ATP vary in various physiological states? How significant is the role of intracellular pH here? In all these studies I will measure non-invasively cellular phosphates by 31P-NMR, oxygen consumption by conventional methods, and mitochondrial redox states by NADH fluorometry. Finally, using the detailed information and concepts developed in the isolated muscles and animal models, I will work to measure relevant energetic parameters by 31P-NMR methods in human limbs, and to develop the concept of a quantitative metabolic stress test.